Wood pulp fibers are valued for their low cost and ease of handling. Wood pulp fibers are also compostable and renewable and can be grown on land typically not suitable for food crops.
Wood pulp fibers are produced by either mechanical pulping or chemical pulping. Wood, which is a composite structure in which mostly cellulosic fibers are held together with hemicellulose and lignin, is broken down during the pulping process so that individual fibers are obtained. Mechanical pulping uses various types of grinding to accomplish this task and most of the hemicellulose and lignin are retained. Chemical pulping, on the other hand, uses chemicals to break the bonds holding the composite structure together and most of the lignin is dissolved and removed.
Pulps are generally characterized in the industry as paper-grade pulp or fluff pulps. Paper-grade pulps, derived from either hardwoods or softwoods, are used in the manufacture of paper, tissue, and packaging. Paper-grade pulps are typically sold in a heavy-weight sheet form similar to cardboard, with densities greater than 0.700 grams/centimeter3. The paper-grade pulp sheets are packaged as discontinuous sheets that are stacked up to form bales.
To make articles with paper-grade pulp, the baled paper-grade pulp is introduced into a hydropulper and a slurry is formed. In the hydropulper, the paper-grade pulp fibers are redispersed in water by aggressive mechanical agitation. Consequently, moisture content of paper-grade pulps is typically disregarded, and thus, the high costs associated with pulp drying are avoided. As a result, paper-grade pulps have a moisture content greater than 10% and in some cases as high as 20-25% or more. Further, the high densities (greater than 0.700 g/cm3) of paper-grade pulp do not pose a problem as the aggressive mechanical agitation easily disperses the fibers into a slurry suitable for paper-making.
Fluff pulps are typically employed in the manufacture of non-wovens. Non-wovens are fibrous webs or fabrics in which the fibers are neither woven nor knitted. Non-wovens are typically made by dry-laying processes, such as airlaying, spunbonding, hydroentangling, etc. As such, and in contrast with paper-grade pulp, fluff pulp has a moisture content of 10% or less. Fluff pulps are typically made from long fibered softwoods, such as loblolly pine. Fluff pulps are dried in a continuous sheet form and wound onto rolls.
Currently available fluff pulp is quite hydrophilic, limiting the uses of fluff pulp to adsorbents, such as diapers, feminine hygiene products, incontinence products, etc. Since fluff pulp is economical, the pulp industry is constantly striving to produce new applications for fluff pulps.